Asymmetric division is the main phenomenon establishing cellular diversity. From a single haploid P (Plus) or M (Minus) cell, S. pombe is able to produce a cell population containing both mating types in nearly equal proportions. This process exploits the intrinsic asymmetry of DNA synthesis to restrain a gene conversion event to one of the two sister chromatids. The genes required in this process are also essential for the organisation and the maintenance of the genome integrity.

The mating type of the cell is determined by the allele present at the mat1 locus: mat1-P in P cells, and mat1-M in M cells. The mat1 allele can be replaced efficiently by genetic information contained in one of the two silent donor cassettes mat2-P and mat3-M. The gene conversion event insures a homogenous mixed population of P and M cells. When the cells are starved, two cells of opposite mating-type mate to form a transitory diploid and produce four spores. When growth conditions are re-established the spores germinate giving rise to four haploid cells (2P and 2M cells), closing the life cycle.

Our previous work has shown that the gene conversion event is induced by a single-strand DNA lesion and involves the DNA replication process. The DNA lesion appears during DNA replication on the DNA strand produced by the lagging replication machinery. This lesion is stabilized, and restricts the gene conversion event to one of the two sister chromatids, during the next round of DNA replication. A direct prediction and extension of this model was supported by experiments inspired from the pioneering approach of Meselson and Stahl (1958), which showed that both DNA strands of the mat1 locus are synthesized de novo in one fourth of the cell population, whereas mat2 and mat3 follow the classical semi-conservative replication pattern in all cells.

Atanas Kaykov (PhD sudent) demonstrated that the imprint is a single-strand DNA-break, with no base pair missing. Several cis-acting elements were identified, in which one allows a fork of replication pause depending on the swi1 gene product. Allyson Holmes, has constructed an inducible system, either controlling the formation or the repair of the break. The "ON" condition will allow sequential analysis of break formation followed by gene conversion. The "OFF" conditions will allow the study of single-strand break repair at a specific site in the genome. Blerta Xhemalce (PhD student) is working on the role of SUMO in the DNA metabolism regulation and genomic stability.